PROJECT SUMMARY The ability to reprogram myriad differentiated cells back into a pluripotent stem cell state highlights the remarkable plasticity of somatic cells. Effort is needed to better understand the inherent plasticity of tissue stem cells as well as the underlying regulatory mechanisms, so that we can learn about how to control cell fates in tissue engineering and regenerative medicine. Epithelial cells are long known to possess inherent plasticity, best manifested by their ability to become mesenchymal cells as well as to revert back to an epithelial state, under appropriate stimuli. This plasticity is important for generating mesenchymal cell types during embryogenesis, and is thought to be required for malignant cancer cells to form distant metastases. Recent studies have inspired the hypothesis that the mechanisms that regulate this plasticity are used in committed tissue epithelial cells to control stem cell fates. The mammary epithelia serve an outstanding model system to test this hypothesis, owing to the well-established methods to prospectively isolate and functionally characterize their residential stem cells. The basal layer of the mammary epithelia harbors multipotent stem cells and cells that can turn into such stem cells under conditions such as transplantation and pregnancy. We will organically combine cutting-edge single-cell analysis with state-or-art technology such as tissue-specific gene knockout and inducible overexpression, in vivo and ex vivo stem cell assays, as well as molecular studies to address the following fundamentally important questions: 1) How many cell type subsets or cellular states exist in the mammary basal cell population and do these states correspond to distinct positions in the spectrum of epithelial/EMT plasticity? 2) Are multipotent stem cells or cells with the capacity to become such stem cells defined in part by their high plasticity, namely the potential to adopt both mesenchymal-like and terminal epithelial fates? 3) What is the key molecular circuitry that regulates such plasticity, and whether/how this circuitry dictates the stay of a basal cell in a stem cell state?